4 The DNA molecule is shaped like a
ladder. The two strands of the ladder
support the steps. Each step is made up of
a pair of chemicals called bases. The secret
of heredity has to do with how these bases
are put in order. The DNA of a human cell
has billions of base pairs. If you stretched
out and connected the DNA in one human
cell, it would stretch for more than 1.5
meters (5 feet).